FindGTK2: Print a message instead of returning if a dependency was not found

Checks are now executed only if target is supposed to be created, not if the
library was not found.

This will help finding out the reason why the tests are not executed on some
build machines.
FindGTK2: Print target info when building the tests

This will help debugging while tests fail on some build machines.
Merge topic 'FindGTK2-targets' into next

ff69cec FindGTK2: Print target info when building the tests
6451989 FindGTK2: Print a message instead of returning if a dependency was not found